Method of laminating polyimide to thin sheet metal

ABSTRACT

An improved method of laminating a metal foil or sheet to a polyimide material is provided. A solution of a precursor of an intractable (i.e. thermosetting) polyimide is applied to a substrate and the solvent is removed to form a dry tack-free film. Thereafter, a solution of a precursor of a thermoplastic polyimide is applied onto the first film of polyimide and the solvent is removed to form a dry tack-free second film. Both films are then cured concomitantly at a sufficiently rapid rate and low temperature to effect substantial imidization of the polyimide precursors of both films without substantial crosslinking or densification of the polyimides in either of the films. Thereafter, a metal sheet or foil is laminated onto the thermoplastic polyimide film according to the following process. The thermoplastic film is contacted with the sheet or foil of metal to be laminated thereto. A first pressure is applied to the metal sheet and the polyimide substrate composite, which pressure is sufficiently low to permit outgassing of any gases trapped or generated within the polyimides and the temperature is increased. The temperature is raised to a level to essentially complete imidization of the polyimides and also to expel any gases contained or generated by said polyimide films. Thereafter, the pressure is increased to a second level or value and the temperature is controlled to a value which temperature is above the T g  of the thermoplastic film. This second pressure is sufficiently high to complete the lamination of the metal sheet or foil to the thermoplastic polyimide.

FIELD OF THE INVENTION

This invention relates generally to laminating of polyimide materials tometal sheets, and more particularly to an improved method of laminatingthin sheets of metal to a dual layer of polyimide cast on a metalsubstrate. In still other aspects, this invention relates to an improvedmethod of curing, i.e. imidizing thermoplastic polyimides.

BACKGROUND OF THE INVENTION

Laminates comprising one or more layers of polyimide and one or morelayers of substrate materials such as metals and alloys may be used fora variety of applications. These applications include structures whichprovide both structural integrity as well as electrical circuitry suchas in circuit boards or the like, and many other uses. When laminatesare to be used for current carrying applications, such as circuit boardsor the like, the metal substrates are selected because of theirmechanical properties, as well as their electrical and/or magneticproperties. At least one of the metal substrates typically is copper ora copper alloy because of the high conductivity of copper and itsalloys. The polyimide layer or layers are selected because of theirdielectric properties.

There are many prior art processes for laminating polyimides to varioustypes of metal sheets or foils. U.S. Pat. No. 4,675,246 to Kundinger, etal. describes various different types of laminate configurations used indifferent types of polyimides and different metal substrates. However,this patent discloses only conventional curing techniques for curing thepolyimide materials. Additionally, U.S. Pat. No. 3,607,387 to Lanza, etal. discloses a thermoplastic polyimide used in contact with a wireconductor such that the circumference is encased. The thermoplasticpolyimide is then encased with an intractable polyimide. This referencedoes not teach a process for lamination of sheets, only a process fordip coating or extrusion coating of wire.

U.S. Pat. No. 4,411,952 to Sasaki et al. describes the use of specificpolyimides formed from reaction of 3, 3', 4, 4'- or 2, 3, 3', 4'-biphenyl tetracarboxylic acids having any one of a variety of diaminebridging units. These precursors are coated and then fully imidized toform a sheet which imidized sheet is then laminated to a conductor foilby conventional heat/pressure processes.

U.S. Pat. No. 4,503,285 to Darms, et al. discloses the use ofpolyamide/polyamide acid copolymers and block copolymers as usefulprecursor solutions for obtaining polyamide/polyimide composite filmswith high adhesion to metal foils. The polyamide/polyamide acids arecoated onto a conducting foil and cured in situ. The foil is thenphotopatterned and etched. The curing is conventional and forms apolyamide/polyimide film.

U.S. Pat. No. 4,543,295 to St. Clair et al. discloses the use of fullyimidized thermoplastic polyimide films placed between metal foils, andthe use of thermoplastic polyamic acid, applied to the fully imidizedpolyimides or metal foils to act as a thermoplastic adhesive. While thispatent discloses the use of linear aromatic polyamic acids andpolyimides as adhesives, it stresses only the importance of adequatethermal treatments to minimize outgassing during lamination processing.Apparently there is no attempt to minimize thermally initiatedinterchain crosslinkage or polyimide densification during eitherpolyimide curing or lamination processing.

U.S. Pat. No. 4,681,654 to Clementi, et al. discloses a method formaking a polyimide based chip carrier by application of either apolyamic acid or, a fully imidized polyimide solution onto a metalcarrier. The polyimide film is then cured by either thermally inducedimidization in the case of the polyamic acid film, or by simple solventevaporation in the case of the preimidized film, so that the metalcarrier can be removed to allow a thin free-standing polyimide film tobe formed. This free standing polyimide film can be bonded to a supportframe or roll carrier for further processing with the use of anadhesive. This adhesive is comprised of either a polyimide, acrylic, orepoxy resin.

U.S. Pat. No. 4,883,718 to Ohta, et al. teaches the use of specificclasses of polyimides as thermoplastic adhesives for obtaining highadhesion forces to metal foils.

U.S. Pat. No. 4,939,039 to Watanabe teaches the synthesis of polyimideshaving low thermal expansion coefficient for direct coating and curingon metal conductors.

U.S. Pat. No. 4,931,310 to Anschel, et al. discloses a technique fortreating the surface of an intractable polyimide to form polyamic acid,which is then rapidly converted to polyimides by IR radiation. Notechniques for lamination is disclosed nor is any technique forselectively imidizing thermoplastic polyamic acid.

IBM Technical Disclosure Bulletin, Vol. 31, No. 11, Apr. 1989, pp. 32-33discloses a technique for imidizing an intractable polyimide to preventthe formation of skin by IR radiation. No lamination technique isdisclosed nor any selective imidization of thermoplastic polyimides.

Whatever technique or materials are used in forming a lamination betweena polyimide material and a metal foil, it is necessary to have a solidcontinuous high strength adherent bond between the polyimide materialand the metal laminates so as to provide the necessary structuralintegrity to any parts formed therefrom thus assuring that the partswill not delaminate. Structural integrity is especially critical incertain applications where the laminated structure serves both as acurrent carrying member and also as a structural member.

A conventional prior technique for bonding laminates such as thosedescribed in certain of the above cited patents includes applying afirst film of thermosetting or intractable polyimide precursor to ametal substrate and thermally imidizing the interactable polyimide. Asecond layer of thermoplastic polyimide precursor is coated over theintractable polyimide and thermally imidized. The dual layer polyimideon the metal substrate is then laminated to a metal foil such as copper.It has been found, however, that the use of conventional oven curing(thermal) techniques and lamination processes produce laminates withinsufficient bond strength between the laminated metal and thethermoplastic polyimide. The reasons for such low bond strength and poorlamination properties are not completely understood. However, in theprior art it was believed due, at least in part, to gas being trappedbetween the thermoplastic polyimide and the sheet metal being laminatedthereto thus preventing good adhesion, and in some instances, resultingin significantly large (i.e. macro) areas of interface which are notbonded at all due to entrapped gases.

SUMMARY OF THE INVENTION

According to the present invention an improved method of laminating ametal foil or sheet to a polyimide material is provided. In oneembodiment, a solution of a precursor of an intractable (i.e.thermosetting) polyimide is applied to a substrate and the solvent isremoved to form a dry tack-free film.

Thereafter, a solution of a precursor of a thermoplastic polyimide isapplied onto the first film of polyimide and the solvent is removed toform a dry tack-free second film. Both films are then curedconcomitantly at a sufficiently rapid rate to effect near completeimidization of the polyimide precursors of both films withoutsubstantial crosslinking or densification of the polyimides in either ofthe films. This is preferably accomplished by using IR radiation in the2.82-3.28 micron range, while maintaining the material at 250° C. orless to prevent crosslinking and other phenomena which could raise theglass transition temperature (T_(g)) of the thermoplastic polyimide, yetpromote imidization. In addition, interdiffusion of polyimide chains atthe intractable polyimide/thermoplastic polyimide interface occursgiving rise to a strongly adhesive polyimide/polyimide interfacial zonewithout the need for additional adhesion promoting techniques.

Thereafter, a metal sheet or foil is laminated onto the thermoplasticpolyimide film according to the following process: The thermoplasticfilm is contacted with the sheet or foil of metal to be laminatedthereto. A first pressure is applied to the metal sheet and thepolyimide substrate composite, which pressure is sufficiently low topermit outgassing of any gases trapped or generated within thepolyimides. A vacuum may also be applied to promote outgassing ofentrapped species. Thereafter the laminating temperature applied to thepolyimides and metal sheet is increased while maintaining the firstpressure. The temperature is raised to a level to essentially completeimidization of the polyimides and also to expel any gases contained orgenerated by said polyimide films. A small amount of crosslinking anddensification of the polyimides in both films, both within each film andbetween the films, will occur during this step and is unavoidable.Thereafter, the pressure is increased to a second level or value and thetemperature is controlled to a value which temperature is above theT_(g) of the thermoplastic film. This will typically be a highertemperature than the first temperature level. This second pressure issufficiently high to complete the lamination of the metal sheet or foilto the thermoplastic polyimide. Following this, the pressure ismaintained until the laminating composite is cooled to below the T_(g)of the thermoplastic polyimide. The resulting structure will be apolyimide core interposed between and bonded to the metal substrates.

In another embodiment, the layer of intractable polyimide can be omittedand the thermoplastic polyimide precursor can be applied directly to asubstrate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the temperature and pressure transits duringthe lamination portion of the present invention; and,

FIG. 2 is a cross-sectional view somewhat schematic, of a laminateformed according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, an improved method of forming astructure of a single or multilayer polyimide formed on a substrate witha copper alloy foil, or other metal sheet laminated thereto is provided,as well as an improved method of curing the thermoplastic polyimide usedtherein. The present method reduces or eliminates previous problemsencountered during conventional laminating processes which in the pastresulted in poor adhesion and indeed significant areas of nonlaminatingor nonbonding between the copper foil and the polyimide layer. Thepresent invention will be described as it is practiced in conjunctionspecifically with laminating a thin foil of copper alloy onto a duallayer of polyimides, i.e. thermoplastic polyimide and an intractablepolyimide which dual layer is bonded onto a stainless steel substrate.It should be understood however that the process is not so limited tothese particular materials but rather it is applicable to the laminationof sheets of various types of metals and materials onto polyimides aswill be readily recognized by those skilled in the art.

A substrate of a sheet of 0.003 inch thick AISI type 302 stainless steelis provided. Onto this sheet of stainless steel is coated a solution ofthe precursor of an intractable polyimide. An example of one suchpreferred intractable polyimide is poly(4, 4'-oxydiphenylene benzene -1,2, 4, 5-tetracarboxylicdiimide) PMDA-ODA, the precursor polyamic acidsolution being available from various commercial sources such as E.I.DuPont de Nemours Co. having a place of business in Wilimington,Delaware 19898, as Pyralin® PI-5878 or Pyralin® PI-2540. PMDA-ODA is anintractable or thermosetting polyimide in that when it is fullyimidized, it does not present a well-defined glass transition (T_(g))temperature and is insoluble in phenolic solvents. The intractableprecursor PMDA-ODA polyamic acid is applied to such a thickness that theresulting film thickness after drying and curing will be about (but notlimited to) 4 microns. Following the coating of the PMDA-ODA onto thesubstrate, it is heated to a sufficient temperature and for a sufficientamount of time to drive off a major portion of the solvent but not tocause any significant imidization or densification of the material.Heating to 85° C. for 15 minutes for the preferred 4 micron thick filmwill normally be sufficient to cause the removal of the solvents so thatthe PMDA-ODA dries to an essentially dry, tack-free film adhering to thestainless steel.

Following this formation of a dry tack-free film, a solution of aprecursor of a thermoplastic polyimide is coated over the intractablepolyimide film. A preferred type of thermoplastic polyimide is DuPontPyralin® PI-2566 poly(4,4'-oxydiphenylene1,1'-hexafluoropropyldiphenylene 3,3'4,4'-tetracarboximide) 6FPDA-ODA.This is applied to a sufficient thickness such that when the film isdried and cured it will have a thickness of (but not limited to) about 2microns. This second coating is also dried to remove a major portion ofthe solvent to produce a dry tack-free essentially nonimidized film.This drying preferably can take place at about 85° C. for about 15minutes. It will be understood that the two polyimide precursors areboth supplied as organic solutions. Depending upon the method chosen forcoating, the desired film thickness, and other parameters, the solutionviscosity can be altered by the addition of solvent as needed.

At this point in the processing, there is a stainless steel substratehaving a first layer of intractable polyimide precursor adhering theretoand a second film of a thermoplastic polyimide film over the intractablepolyimide precursor film.

The two polyimide precursor films are then subject to a curing processwhich will rapidly imidize the polyimides of both layers without causingsubstantial crosslinking. The terms "imidization" and "curing" ofpolyimides as used herein are synonymous and describe the chemicalreaction which forms polyimides from polyamides. When polyamic acids arethe beginning polyamide, water is liberated during imidization. Whenethyl esters of polyamic acids are the beginning polyamide, ethanol isliberated during imidization. This is preferably performed by exposingthe film to infrared radiation (IR) in the near infrared region (i.e. awavelength of about 2-3 microns) while heating the films to about 250°C. The process takes place in an oven heated by infrared radiation whichhas the temperature controlled to 250° C. The IR radiation not onlyheats the furnace and promotes outgassing of the polymer but alsosignificantly increases the rate of reaction for imidizing the polyimideprecursors, without inducing any significant crosslinking anddensification. Interchain polyimide crosslinking is known to result fromthermal input. Interchain imidization between neighboring chain polyamicacids may occur during polyimide curing. Because long thermal cureprocesses at high temperature are required for full imidization, muchmore solvent is removed prior to forming a completely imidized film.Solvent removal places neighboring chains in closer proximity andincreases the probability of transimide formation. In fully curedpolyimide films, thermally activated C-N bond-breaking within the imidefunctionality gives rise to nitric radicals. These radicals, whenproduced on neighboring polyimide chains can react to produce interchaintransimides. Because this process is thermally activated, increasingthermal treatment (time and/or temperature) will result in some increasein transimide polyimide. In the case of thermoplastic polyimides thisresults in increasing polyimide T_(g). This reaction rapidly increasesin rate at T>250° C., although large amounts of transimides are notbelieved to be formed since reimidization to the original polyimide isstrongly favored for steric reasons.

A more detrimental phenomenon (from the standpoint of raising the T_(g))is thermally activated interchain crosslinking which can also occurthrough the R₁ bridging portion of the monomer for thermoplasticpolyimides of the form: ##STR1## or i.e. a single bond); preferably C₃F₆ ; x is 1-10;

R₃ is a linear or branched alkyl group which is partially or completelyhalogenated or an aromatic or heteroaromatic group;

R₂ is a divalent aromatic group, including the structure: ##STR2## Ar=isa trivalent aromatic group, including the structure: ##STR3##

In the case of 6FPDA-ODA, R₂ is: ##STR4## the R₁ group ishexafluoropropyl (C₃ F₆) and crosslinking is presumed to be the resultof R₁ conversion to a trifluoroethyl radical and accompanying loss oftrifluoromethyl radicals. This process is analogous to the formation ofnitric radicals discussed previously. Here, trifluoroethyl radicals onneighboring polyimide chains react producing an interchain crosslinknetwork. This network raises the T_(g) of the thermoplastic polyimide.

Thermal treatment of thermoplastic polyimides can also raise the T_(g)of the respective polyimide by causing densification of the polymermatrix. Long thermal treatments result in densely packed and intertwinedpolymer chains promoted by slow rates of imidization and solventremoval. Therefore, a cure method capable of effecting imidizationrapidly upon a more highly solvated and less closely spaced polymer canresult in a more amorphous, less densely packed polyimide with enhancedthermal reflow properties (lower Tg).

Thus, a method of polyimide curing which minimizes thermal input to thepolyimide while maximizing the rate of imidization and outgassing ishighly desirable. A preferred cure process uses infrared wavelengthphotons to dramatically increase the rate of imidization. Thermalassistance is also provided to increase the rate of polymer outgassing.The infrared wavelength chosen is determined from the N--H stretchingband of the IR spectrum of the precursor polyamic acid. A wavelengthchosen near the absorption maximum (N--H) is selected for maximumquantum yield of imidization. For most polyamic acids this absorbanceband is located between 3540 cm⁻¹ to 3050cm⁻¹ (2.82-3.28 μm) in theelectromagnetic spectrum and this range is preferred for curing6FPDA-ODA. The wavelength or wavelength range selected may fall outsidethe preferred range depending upon the absorption characteristics of thepolyamic acid being imidized and upon the emission characteristics ofthe exposure system. At the wavelength or within the wavelength range ofexposure it is preferred that there be little or no additionalabsorbance by the polyimide which may give rise to crosslinkingreactions. That is, those wavelengths capable of initiating bridginggroup (R₁) crosslinking reactions preferably should be excluded from theexposure radiation. The temperature of the film should be kept low sinceincreased temperatures will promote (R₁), group crosslinking dueprincipally to thermal energy which will occur with the imidization ofthe thermoplastic polyimide precursor. This temperature should be as lowas possible, however, sufficiently high to facilitate outgassing, e.g.not above about 250° C.

Thus, by utilizing IR in the curing process, it is possible to quicklyimidize both polyimides to a substantial extent without significantcrosslinking or densification. This is important, especially withrespect to the thermoplastic polyimide since as crosslinking and/ordensification progresses, the T_(g) of the thermoplastic polyimiderises, and in the subsequent lamination process it is necessary to heatthe thermoplastic polyimide to above the T_(g) to effect bonding of themetal foil to the polyimide and for several reasons it is desirable tobond at as a low a temperature as possible. Hence, by avoiding anysubstantial crosslinking or densification at this stage of the process,the bonding can be accomplished at a low temperature.

After about 4 minutes of exposure to the IR radiation at 250° C. for theabove noted film thickness, both the polyimides PMDA-ODA and 6FPDA-ODAare essentially fully imidized. However, because of the relatively lowtemperature (250° C.) and the relatively short period of time (4minutes), no substantial crosslinking of the polyimides occurs.

Following the IR cure the layer of copper foil which in the preferredembodiment is a beryllium-copper alloy, such as Alloy 3 which is sold byBrush Wellman having a place of business at 17876 St. Clair Avenue,Cleveland, Ohio 44110, is then laminated to the thermoplastic polyimidelayer. The copper alloy foil may have an applicable surface texturingtreatment to improve adhesion characteristics such as those imparted bythe JTC treatment performed by Gould Inc., Foil Division, having a placeof business in Eastlake, Ohio 44094. This surface texturing treatment isthe result of electrochemical deposition of Cu onto the to-be-bondedBe/Cu surface. The preferred treatment provides a surface layer which isdendritic in nature, with these dendrites providing increased surfacearea and texture for interlock with the thermoplastic polyimide. It ispreferred that the heights of these dendrites not exceed the thicknessof the thermoplastic polyimide layer.

A stack comprised of the stainless steel with the polyimides bondedthereto is placed in a laminating press and the copper foil is placedover the thermoplastic polyimide and the pressure is raised to about 10psi. Any conventional laminating press can be employed. One particularlyappropriate press is a Model MTP-24 press manufactured by TetrahedronAssociates, Inc. having a place of business at 5060 Convoy Street, SanDiego, California 92111, which is a press having 24"×24" platens.

As shown in FIG. 1, the temperature is then raised during a period ofabout 40 minutes to about 340° C. During this step in the processing,any gas that is trapped in the polyimides or which is formed by virtueof this heating up to 340° C. will be driven to the interface of thecopper foil and the thermoplastic polyimide. Since the pressure isrelatively low, i.e. about 10 psi, the gas can escape at the interfaceand not be trapped thereat. A vacuum may be applied during this step inthe process to aid in degassing. After about 40 minutes when the heathas been elevated to about 340° C., essentially all of the trapped andformed gas has been outgassed.

At this point, both of the polyimide structures have crosslinked to someextent internally and at their interface. But the T_(g) of thethermoplastic is still relatively low and it will still flow well forbonding at temperatures well below 400° C. The pressure is then rampedup quickly to about 675 psi and the temperature is concurrently raisedto about 360° C.

As indicated above, because of the very rapid initial imidization of thetwo polyimides at low temperature of 250° for 4 minutes which did notproduce any substantial crosslinking or densification, the temperaturetransit from ambient to 340° results in a structure in which thethermoplastic layer of polyimide has a T_(g) substantially less than360° C. since only a small amount of crosslinking occurs during thetemperature transit to 340° C. Thus, when the temperature is raised to360° C., which is well above the T_(g) of the thermoplastic polyimideand a high pressure of 675 psi is applied concurrently, thethermoplastic polyimide will flow sufficiently to form a strongsubstantially continuous bond between the copper alloy foil and thethermoplastic polyimide. Also, during both heating steps, somecrosslinking and chain entanglement occurs between the intractablepolyimide and the thermoplastic polyimide at their mutual interface.This crosslinking and chain entanglement aids in the formation of astrong intractable polyimide/thermoplastic polyimide interface. From thestandpoint of mechanical peel and adhesion performance, the finishedlaminate behaves as a single layer of polyimide bonded to stainlesssteel substrate on one side and copper alloy foil on the other.

The temperature and pressure are maintained at these elevated values for30-45 minutes at which time the temperature is gradually reduced tobelow about 50° C. at which time the pressure will be reduced andremoved. The heating at 360° C. for 30-45 minutes will cause furthercrosslinking and densification of the polyimides to continue, formingsolid polyimide having good structural and dielectric properties.

The end product formed by the above described process has aconfiguration as shown in FIG. 2. The structure includes a base ofstainless steel 10, a layer of intractable polyimide 12 bonded thereto,a layer of thermoplastic polyimide 14 bonded to and crosslinked with thepolyimide 12, and a beryllium copper alloy foil 16 mechanically bondedto the thermoplastic polyimide 14.

It has been found that a structure laminated according to this processhas a very high bond strength in the critical copper to polyimide bondwhich was formed by the lamination step. Indeed, the adhesion of thecopper foil to the polyimide consistently exceeds 4 pounds per inch asmeasured by a standard 90° peel test. Further, no macroscopic nonbondedareas where there is adhesion loss due to entrapped gases were found ascompared to their presence in a significant number of cases when theprior techniques of imidizing and lamination were utilized.

As indicated above the present lamination process is not limited to thespecific materials and specific temperatures and pressures described.This process can be used in any cases where a layer of intractablepolyimide is bonded to a substrate and a layer of thermoplasticpolyimide is applied over the intractable polyimide and a metal foil ormetal sheet is laminated to the thermoplastic polyimide. In even broaderaspects, this bonding technique can be used when a layer ofthermoplastic polyimide is applied directly to a substrate (without theuse of an intractable polyimide layer). The significant features arethat the thermoplastic polyimide must be cured sufficiently rapidly tocause substantial imidization without any substantial crosslinking ordensification and thereafter lamination takes place beginning at a lowpressure and a temperature ramped upwardly to cause essentially completeoutgassing, followed by an increase in pressure and at a temperatureabove the T_(g) of the thermoplastic to cause laminar bonding.

While several embodiments of the present invention have been shown anddescribed various adaptations and modifications can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A method of laminating a metal sheet to a polyimide material comprising the steps of:applying a solution of a precursor of an intractable polyimide to a substrate, and removing the solvent to form a dry tack-free first film; applying a solution of a precursor of a thermoplastic polyimide material to said first film and removing the solvent to form a dry tack-free second film; curing both films sufficiently rapidly to effect substantial imidization of each film without substantial crosslinking or densification of either of the polyimide films internally; thereafter laminating a metal sheet onto said thermoplastic polyimide according to the following process:(a) contacting the imidized thermoplastic polyimide with a sheet of metal to be laminated thereto; (b) applying a first pressure to said sheet of metal and said polyimide substrate composite which is sufficiently low to permit outgassing of any gas trapped or generated within said polyimides; (c) increasing the temperature of the polyimides while maintaining said first pressure to thereby essentially completely imidize said polyimides and expel gases contained in said polyimides; (d) increasing the pressure to a second pressure value and heating to a temperature which is above the T_(g) of said thermoplastic polyimide to complete lamination of the metal to the polyimide; and (e) thereafter cooling to ambient and removing the pressure.
 2. The method as defined in claim 1 wherein the cure of the polyimide to effect substantial imidization includes the use of infrared energy.
 3. The method as defined in claim 1 wherein said metal sheet is a copper alloy.
 4. The method as defined in claim 3 wherein the copper alloy is beryllium copper.
 5. The method as defined in claim 1 wherein said temperature is raised to a second value when said pressure is raised to said second value.
 6. The method as defined in claim 1 wherein said substrate is a steel alloy.
 7. The method as defined in claim 2 wherein said cure which includes infrared energy is performed at a temperature sufficiently low to prevent any substantial crosslinking or densification of the thermoplastic polyimide, with said infrared energy including radiation in the band width of 2.82 to 3.28 microns.
 8. The method as defined in claim 7 wherein said cure temperature does not exceed about 250° C.
 9. The method as defined in claim 2 wherein said first pressure is not greater than about 10 psi and the temperature is increased while at said first pressure to about 340° C.
 10. The method as defined in claim 9 wherein the second pressure value is about 675 psi.
 11. The method as defined in claim 10 wherein the temperature is raised to about 360° C. during the time the second pressure is applied.
 12. The method as defined in claim 1 wherein the thermoplastic polyimide precursor has the structure: ##STR5## or --(i.e. a single bond);x is 1-10 R₃ =is a linear or branched alkyl group which is partially or completely halogenated or an aromatic or heteroaromatic group; R₂ =is a divalent aromatic group; Ar=is a trivalent aromatic group.
 13. The method in claim 12 wherein:R₁ is: C₃ F₆ ; R₂ is; ##STR6## Ar is: ##STR7##
 14. The method as defined in claim 1 wherein a vacuum is applied during at least a portion of step (c).
 15. A method of laminating a metal sheet to a polyimide material comprising the steps of:applying a solution of a precursor of a thermoplastic polyimide material to a substrate and removing the solvent to form a dry tack-free film; curing said film sufficiently rapidly to effect substantial imidization of said film without substantial crosslinking or densification of the film internally; thereafter laminating a metal sheet onto said thermoplastic polyimide film according to the following process:(a) contacting the imidized thermoplastic polyimide with a sheet of metal to be laminated thereto; (b) applying a first pressure to said sheet of metal and said polyimide substrate composite which is sufficiently low to permit outgassing of any gas trapped or generated within said polyimide; (c) increasing the temperature of the polyimide while maintaining said first pressure to thereby essentially completely imidize said polyimide and expel gases contained in said polyimide; (d) increasing the pressure to a second pressure value and heating to a temperature which is above the T_(g) of said polyimide to complete lamination of the metal to the polyimide; and (e) thereafter cooling to ambient and removing the pressure.
 16. The method as defined in claim 15 wherein the cure of the polyimide to effect substantial imidization includes the use of infrared energy.
 17. The method as defined in claim 15 wherein said metal sheet is a copper alloy.
 18. The method as defined in claim 17 wherein the copper alloy is beryllium copper.
 19. The method as defined in claim 15 wherein said temperature is raised to a second value when said pressure is raised to said second value.
 20. The method as defined in claim 15 wherein said substrate is a steel alloy.
 21. The method as defined in claim 16 wherein said cure which includes infrared energy is performed at a temperature sufficiently low to prevent any substantial crosslinking or densification of the polyimide, with said infrared energy including radiation in the band width of 2.82 to 3.28 microns.
 22. The method as defined in claim 21 wherein said cure temperature does not exceed about 250° C.
 23. The method as defined in claim 22 wherein said first pressure is not greater than about 10 psi and the temperature is increased while at said first pressure to about 340° C.
 24. The method as defined in claim 23 wherein the second pressure value is about 675 psi.
 25. The method as defined in claim 24 wherein the temperature is raised to about 360° C. during the time the second pressure is applied.
 26. The method as defined in claim 15 wherein the polyimide has the structure: ##STR8## or --(i.e. a single bond);x is 1-10 R₃ is a linear or branched alkyl group which is partially or completely halogenated or an aromatic or heteroamatic group; R₂ is a divalent aromatic group; Ar is a trivalent aromatic group.
 27. The method in claim 26 wherein:R₁ is: C₃ F₆ ; R₂ is: ##STR9## Ar is: ##STR10##
 28. The method as defined in claim 15 wherein a vacuum is applied during at least a portion of step (c). 